Hammer toe is a deformity of the toe that affects the alignment of the bones adjacent to the proximal interphalangeal (PIP) joint or distal interphalangaeal (DIP) joint. Digital deformities of the fingers and toes are some of the most common conditions encountered by extremity specialists. These digital deformities can cause pain and lead to difficulty in walking, grasping or holding items and even wearing shoes. Examples of such deformities are popularly known as mallet finger, jersey finger, hammertoe, claw toe and mallet toe, as well as many others, indicative of several different pathologies.
Depending on the severity of the deformity, surgery may be required to correct the deformity by fusing one or both of the joints together, including the proximal interphalangeal (PIP) joint and the distal interphalangeal (DIP) joint. In order to prevent recurrence of the deformity and ensure the success of the surgical procedure, a proximal interphalangeal (PIP) joint or distal interphalangeal (DIP) joint arthrodesis is typically performed. The most commonly used hammertoe procedure is that which was described by Post in 1895 and is referred to today as the Post Arthroplasty or Post Procedure. It involves resecting (removing) the knuckle of the toe at the level of the proximal interphalangeal joint (PIPJ). This joint is the joint closest to the point where the toe attaches to the foot. Typically the Post Procedure will be performed in conjunction with a tendon release on the top (extensor surface) of the foot. The combination of these two procedures results in a toe that will lay flat avoiding direct pressure from the shoe. In the case of a mallet toe or claw toe, the Post Procedure may be performed with or without the tendon lengthening. The PIPJ is aligned with the rest of the toe in a corrected anatomical position and maintained in place by the use of a 0.045 or 0.062 inch diameter Kirschner wire (K-wire) driven across the joint. Initially, the K-wire is placed from the PIPJ through the tip or end of the toe. It is then driven in retrograde fashion into the proximal phalanx. The exposed K-wire is bent to an angle greater than 90 degrees, and the bent portion which is external of the body is cut. Normally a plastic or polymeric ball is placed over the exposed end of the K-wire to protect the patient. The K-wire normally remains in the foot of the patient until the PIP and/or DIP joints are fused in approximately 6 to 12 weeks. This conventional treatment has several drawbacks such as preventing the patient from wearing closed shoes while the K-wire is in place, and the plastic or polymeric ball may snag on some object due to it extending from the tip of the toe resulting in substantial pain for the patient.
There is a fairly high incidence of malunion (healing crooked) with traditional K-wire fixation. Other possible drawbacks of K-wire reported in the literature is that it must be removed which may cause additional pain for the patient along with the reported concern of infection at the insertion point of the toe.
Yet other conventional implants, in lieu of a K-wire, include threaded screws that are disposed within the adjacent bones of a patient's foot or nitinol implants that expand due to the rising temperature of the implant to provide outward forces on the surrounding bone when installed. However, the temperature sensitive material of a nitinol implant may result in the implant deploying or expanding prior to being installed, which requires a new implant to be used. Recently, PEEK polymer implants have been used to accomplish the same fixation of the deformity but these implants do not create bone ingrowth or upgrowth on the implant, which may lead to rotation of the implant and not accomplish the fusion required to correct the deformity over time for the patient. Accordingly, an improved implant for treating toe deformities is desirable.